Marine & Environmental Sciences Faculty Articles



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Functional Ecology



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Coral reef fishes, Ecosystem function, Group size, Herbivory, Predation risk, Predator-prey interactions


  1. Fear of predators (‘fear effects’) is an important determinant of foraging decisions by consumers across a range of ecosystems. Group size is one of the main behavioural mechanisms for mitigating fear effects while also providing foraging benefits to group members. Within coral reef ecosystems, fear effects have been shown to influence the feeding rates of herbivorous fishes, a key functional group that prevents macroalgal overgrowth. Yet, how fear effects and group size interact to shape macroalgal removal on coral reefs remains unclear.
  2. Here, we conducted field-based experiments using models of a common piscivorous fish, the leopard coral grouper Plectropomus leopardus and a series of macroalgal Sargassum ilicifolium assays positioned at increasing distances from the models (1, 2, 3 and 4 m) on two coral reefs in Singapore to investigate how acute fear effects shape the intensity of herbivory, and whether these effects were influenced by variation in the group size of herbivorous fishes feeding on the assays.
  3. We found acute fear effects strongly influenced the foraging behaviour of herbivorous fishes over small spatial scales. Rates of Sargassum biomass removal, feeding rates and the total number of individual feeding events were all lower near the predator model. These effects dissipated rapidly with increasing distance from the predator model and were undetectable at a distance of 4 m. We also found generally larger group sizes of herbivorous fishes further from the predator model, presumably reflecting decreased risk. Furthermore, the number of individual bites/event increased significantly with increasing group size for two common browsing fishes, Siganus virgatus and Siganus javus.
  4. Our findings highlight that acute fear effects influence the distribution and intensity of herbivory over small spatial scales. Fear effects also interacted with herbivore group size resulting in changes in the number of individual feeding events and bite rates that collectively shape the realized ecosystem function of macroalgal removal on coral reefs. Group size is an important context-dependent factor that should be considered when examining fear effects on coral reefs.







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R code and data associated with the manuscript are at (Bauman et al., 2021).

Additional Comments

We acknowledge funding from the AXA Fellowship (A.G.B.: 154-000-649-507) and theNational Research Foundation, Prime Minister's Office, Singapore under the Marine Science Research and Development Programme (MSRDP-P03). All research carried out abided by local laws and was done with permission of the Singapore government through the National Parks Board (Permit #NP/RP15-009c). Logistics and field support was provided by Dolphin Explorer.

Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

Peer Reviewed